Access to medical marijuana appears to have saved thousands of lives over the past few years by reducing accidental overdose deaths from drugs like Vicodin, Percocet and OxyContin, a new study says.

States with legalized medical marijuana saw, on average, 1,700 fewer deaths a year from prescription drugs than they would have otherwise, says the study led by researchers from the Johns Hopkins Bloomberg School of Public Health and the Philadelphia Veterans Affairs Medical Center. Critics call the study flawed, and complain it makes "sweeping" conclusions not supported by the data.

Prescription painkillers are among the most abused prescription medicines in the U.S., federal officials say, responsible for the deaths of more than 15,000 Americans annually. Advocates of medical marijuana say pot is a far safer and more effective alternative, although there's still little research to back that up. The study's authors say their groundbreaking research indicates public health officials need to start looking more closely at the potential benefits of marijuana for managing pain.

"It suggests the potential for many lives to be saved," said study senior author Colleen L. Barry, an associate professor in the Department of Health Policy and Management at the Bloomberg School. "We can speculate … that people are completely switching or perhaps supplementing, which allows them to lower the dosage of their prescription opioid."

Twenty-three states and the District of Columbia now permit some form of medical marijuana. Barry said the study doesn't explain exactly why prescription drug overdoses dropped by 25% in states that permit medical marijuana, but raises important questions. Federal officials are in the process of making it harder for patients to legally acquire large qualities of certain opioid drugs like Vicodin. Opioid drugs like Vicodin are highly addictive, and patients who get hooked have to take increasingly higher doses to feel the drug's effects.

"There's a lot of rethinking about relative harms and relative benefits right now," Barry said. "Medical marijuana is not susceptible to unintentional overdose … What we don't know, and that's because we haven't had enough research done, is how good of a job medical marijuana does for people with chronic pain."

Kevin Sabet, director of the Drug Policy Institute at the University of Florida College of Medicine, said he has many concerns about how the study's authors collected and analyzed the data. He said they failed to differentiate between states with strict and lax medical marijuana laws, and didn't examine emergency-room admission and prescription data, and failed to see what impact methadone clinics might have had. He said it's hard to believe there was such an across-the-board reduction in predicted deaths.

"In today's supercharged discussions, it could be easily misunderstood by people," he said of the study, which he faulted for drawing distinct conclusions based on limited data. "There may be promise in marijuana-based medications but that's a lot different than 'here's a joint for you to smoke.'"